Recently, electrophotographic image formation using electrostatic image developing toners (hereinafter, also denoted simply as toners) has been applicable to full-color prints as well as monochromatic prints as typified in conventional documentation. Such full-color image forming apparatuses, which can make printed sheets as required on demand without making printing plates, as in conventional printing, have been employed mainly for short-run printing in which orders for some hundreds sheets of print is often received, as described in, for example, JP-A 2005-157314 (hereinafter, the term JP-A refers to Japanese Patent Application Publication).
Thus, the field of performing image formation through an electrophotographic system, such as print making having been conducted in a printing factory, has rapidly expanded, while performance of stable print making is required for printers, even if image formation is conducted under some hard environment. For instance, a print making environment in a printing factory is said to be markedly severe, as compared to print making in the office. Further, there can be much anticipated cases of urgent print order and a high-performance toner which can display stable electrostatic-charging capability even under hard environments is desired in order to achieve print making of the required number of sheets within a limited time.
As described above, a toner which exhibits superior electrostatic-charging performance, that is, charge generating capability and charge holding capability even under hard environments of print making, for example, under a high temperature and high humidity condition or under low temperature and low humidity condition, can achieve stable print-making at beautiful-finished levels. Making prints for color catalogs or advertisements demands a toner capable of performing faithful color reproduction. In full-color image formation, mono-color toner images of yellow, magenta and cyan are superimposed to reproduce all color tones and techniques for enhancing color tones of these toners as bases become important points. Examples of such a technique for enhancement of toner color tone include a toner improvement technique by the use of a copper phthalocyanine compound, as described, for example, in JP-A 5-239368 and a toner designation technique for enhancement of color reproduction of photographic images such as color gravures, as described in, for example, JP-A 2006-63171.
Making prints by use of toners disclosed in the foregoing patent documents enabled to make prints of good color tones without color contamination under air-conditioned environments, such as business offices. However, prints which were made under high temperature and high humidity or low temperature and low humidity tended to cause difficulty in obtaining good color tone. It is thought that environmental influences renders it difficult to achieve stable electrostatic-charging and an insufficiently charged toner cannot achieve smooth transfer so that a given amount of a toner is not transferred, resulting in disruption of color balance of the toner image, adversely affecting print image quality. Thus, toner transfer of a given amount often becomes difficult under hard conditions for image formation such as high temperature and high humidity, or low temperature and low humidity, rendering it difficult to maintain favorable color balance of a toner image. Environmental effects rendering it difficult to achieve stable charging is disadvantageous for print factories having many chances of making color prints under hard environments.